The printing plate precursor for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required accompanied with the digitization of printing data. Recently, a so-called processless printing plate precursor has been noticed which can be applied to a printing press employing a direct imaging (DI) process and does not require development by a special developing agent.
Herein, an outline of a printing press as one of the printing press employing a direct imaging (DI) process, disclosed in Japanese Patent O.P.I. Publication Nos. 5-77385, which is equipped with a plate cylinder, a pressure cylinder, an inking unit, and an image formation unit will be explained below employing FIG. 1.
FIG. 1 shows a schematic view of a printing press employing a processless printing plate precursor. Numerical number 1 shows a printing press. Numerical number 101 shows an image recording device, and numerical number 2 shows a printing section. The printing section 2 comprises an inking unit 201 and a dampening water unit 202. The inking unit 201 is an apparatus which supplies ink to a planographic printing plate precursor on which an image has been formed according to the image recording device 101 on a plate cylinder 102, and the dampening water unit 202 is an apparatus which supplies dampening water to the planographic printing plate precursor. Numerical number 102a shows a cramp for fixing a planographic printing plate precursor on the plate cylinder 102.
The plate cylinder 102 can be rotated by means of a driving device not illustrated such as a motor device not illustrated. A blanket cylinder 103, which receives an ink image formed on the planographic printing plate, is provided under the plate cylinder 102, and a pressure cylinder 104, which applies pressure to a paper sheet, and transfers to the paper sheet an ink image transferred on the blanket cylinder 103, is provided under the blanket cylinder 103. Numerical number 105 shows a paper sheet feeding section, numerical number 106 a chain gripper for transporting a paper sheet with the transferred image, and numerical number 107 a paper exit unit. Numerical number 108 shows an enclosure of the printing press 1, and numerical number 109 a paper sheet feeding device.
As a substrate of a planographic printing plate precursor as a processless plate precursor applied to a printing press having the DI capability, the same aluminum plate as used in a conventional PS plate is considered, but processless plate precursor employing a flexible substrate is sought in view of freedom of the precursor constitution, cost reduction, and adaptability to full automation of the DI printing press.
A DI printing press QUICK MASTER 46DI produced by Heiderberg Co., Ltd. holds a planographic printing plate precursor employing a flexible substrate in the roll form on the unrolling spool side within the plate cylinder, unrolls the precursor to supply it on the plate cylinder, and winds the precursor around the winding spool within the plate cylinder, whereby the plate precursor is changed to another plate precursor.
In the above printing press, the precursor is fixed on the plate cylinder by applying tension to the precursor between the unrolling spool and the winding spool, as disclosed in Japanese Patent O.P.I. Publication No. 7-101044.
Since the planographic printing plate precursor exclusive for the DI printing press QUICK MASTER 46DI has a thickness of 200 μm, such a tension neither elongates nor displaces the precursor. However, only a short length of a planographic printing plate precursor, i.e., 30 by plate number, can be held in the roll form within the plate cylinder due to its large thickness.
A DI printing press is suitable for printing a small volume of copies. Therefore, the planographic printing plate precursor for it is used up in a short time, and a frequency of exchange to a new a planographic printing plate precursor roll is high. Reduction of the thickness of a planographic printing plate precursor can lengthen a planographic printing plate precursor in the roll form held within the plate cylinder, and can also contribute to cost reduction of the planographic printing plate precursor. However, a conventional method of fixing a planographic printing plate precursor on a plate cylinder of a printing press has problems in that elongation and displacement of a planographic printing plate precursor occurs during printing.
As a method of fixing a thinner printing plate precursor on a plate cylinder of a printing press, various methods are proposed in Japanese Patent O.P.I. Publication No. 11-28802. One of the methods is a method which provides a printing plate precursor on a plate cylinder having suction orifices uniformly distributed in the surface and sucks air through the orifices to fix the printing plate precursor on the plate cylinder under reduced pressure. However, reduction of the printing plate precursor thickness causes impermissible deformation at the orifice portions which may result in lowering of image quality and printing quality. In order to prevent such a deformation, a method is proposed which reduces a diameter of the orifices to for example, not more than 1 μm, however, such small orifices increase loss of applied pressure, and are difficult to effectively fix the printing plate precursor on the plate cylinder.
Another one is a method employing a planographic printing plate precursor having on the rear surface a heat sensitive adhesion layer whose adhesion force is reduced by heat application.
In this method such a planographic printing plate precursor is fixed through the adhesion layer on a plate cylinder having a heating means capable of heating the plate cylinder surface so that the heat sensitive layer is in contact with the plate cylinder, and after completion of printing, the plate cylinder surface is heated to reduce the adhesion force of the heat sensitive adhesion layer whereby the planographic printing plate is peeled from the plate cylinder. However, this method has problem in that after peeling of a planographic printing plate used for printing, the adhesive layer remains on the surface of the plate cylinder. Particularly in a DI printing press frequently exchanging a planographic printing plate precursor, the adhesion layer is piled on the plate cylinder surface, resulting in a serious problem.
In order to completely remove such a adhesion layer remaining on the plate cylinder surface, the plate cylinder surface is washed through a cleaner. This not only prolongs time for planographic printing plate precursor exchange but also brings about cost increase due to use of consumables.
As a method for heating the plate cylinder surface, a method employing a heater is proposed. The heater cannot promptly elevate temperature of the plate cylinder surface in view of the heat content of the whole plate cylinder. This requires standby until the plate cylinder surface elevates to a predetermined temperature, resulting in prolongation of time for planographic printing plate precursor exchange.
As described above, the planographic printing plate precursor fixing methods proposed so far are not satisfactory, and a method has not been developed which firmly fixes on a plate cylinder without being out of position a thinner planographic printing plate precursor, which is suitable particularly for full automation or labor saving of a DI printing press.